1. Field of the Invention
This invention relates generally to a system and method for controlling a tendon-driven manipulator using closed-loop control. In particular, the joint angle and torque in the joints of the manipulator are controlled in such a way that it reduces coupled transient joint motions and allows for independent motion of the joints.
2. Discussion of the Related Art
Dexterous robot systems typically comprise robot arms and hands that operate to grasp and manipulate objects or parts during assembly or other applications. The term “robot manipulator” is used to describe all or part of one or more robot arms and hands. Tendon-driven robot manipulators are robot manipulators that are actuated using tendons or cables that allow the actuators to be located outside of the part of the manipulator being controlled. Tendon transmissions are frequently used to actuate distal joints in robot manipulators. They can improve strength-to-weight ratios by allowing actuators to be located closer to the base of the manipulator. They also give the mechanical designer more flexibility in actuator selection. For example, when the manipulator is a tendon-driven finger in a humanoid robotic hand, the actuators are typically located in the forearm area of the robotic arm. In this case, tendons extend from the forearm actuators to the fingers, where they are attached.
It is sometimes the case that the number of tendons in a tendon-driven manipulator is associated with a corresponding motion by multiple tendons. This is the case with “n+1” tendon arrangements where n+1 tendons bi-drectionally actuate n joints. n+1 tendon arrangements are useful because they allow bi-directional joint actuation with a minimum number of actuators. However, this presents an extra control burden because the resulting physical coupling between tendons and joints must be decoupled in the control system. During control, the actuators must move multiple tendons so that the tendons do not stretch or break as the manipulator joints move. For one joint motion, there is a plurality of tendon motions that are required to make the joint motion. Thus, there is a complex motion between joint motion and tendon motion.
Known control schemes for controlling the actuators that actuate the tendons are typically hierarchical in nature. Each actuator is associated with a single controller that controls the position of a single tendon or the force applied by a single tendon. A higher level controller sends commands to each of the single-tendon controllers so as to realize a particular joint motion or torque. When this hierarchical approach to control is used, the dynamics of the low-level controllers cause coupled joint motions. In other words, because each low-level controller controls a single tendon, and the motion of a single tendon causes multiple joints to move, the dynamics of the low-level controllers cause coupled joint motions. As a result of this coupling, multiple joints may move when motion of only a single joint is desired. Thus, undesired motions occur in the robotic manipulator.
It is frequently desirable to control joint compliance and torque in tendon-driven manipulators. This is particularly true for robot hands where the tendon driven manipulators are fingers. For these applications, precise control of the dynamics of the forces applied to the environment by the fingers is paramount. For example, compliance control in the joints of robot fingers is an effective way to wrap the fingers around an object, thereby maximizing the contact surface area and improving the grasp. Alternatively, a robot may grasp and lift an object more robustly when the contact forces are accurately controlled. In these applications, it is essential to control joint positions and forces independently and in a dynamically accurate way.